1. Field of the Invention
The present invention relates to a method for controlling focus in an optical disk recording and reproducing device which performs a reproducing operation of a signal recorded on an optical disk using laser light irradiated from a laser diode incorporated in an optical pickup device or a recording operation of a data signal to the optical disk.
2. Description of the Related Art
An optical disk recording and reproducing device has been made commercially available which performs a reproducing operation of a signal recorded on an optical disk using laser light irradiated from a laser diode and a recording operation of a signal to the optical disk. A typical optical disk recording and reproducing device uses an optical disk which is called a “CD” and another typical optical disk recording and reproducing device uses an optical disk which is called a “DVD”.
Because there is a strong demand for recording a larger amount of signals on an optical disk, currently, there is a shift from CD to DVD. Among DVD disks, an optical disk called a dual layer disk has been developed in which two recording layers are provided instead of one recording layer.
A technique has been developed in which laser light irradiated from an optical pickup device is focused on each signal layer in an optical disk recording and reproducing device which uses the dual layer disk.
The operation to control focus for focusing the laser light irradiated from the optical pickup device on two signal layers assuming that the placement of the recording layers of the optical disk is accurate, that is, the physical characteristics of the optical disk conform with the standard.
An optical disk of a DVD system is created by adhering two disks of a predetermined thickness (0.6 mm). As a result, although it is possible to achieve approximate uniform thickness for a cover layer provided between the surface of the optical disk and the first recording layer, the thickness of a spacer section which is the adhering portion of the two disks, that is, the spacer section provided between the first and second recording layers may not be uniform and may vary from one disk to another. As a result, there is a problem in that it is not possible to accurately perform a focus servo operation with an amount of defocus which is set with respect to the first and second recording layers.
In addition, in each optical disk, the thickness of the space section is not uniform over the optical disk. In particular, the thickness of the spacer section may vary significantly between an inner side and an outer side. Thus, there is a problem in that it is not possible to accurately perform a focus servo operation on the overall optical disk.